As of the 4th June this year, Bayo has sold 1 million, might want to adjust the OP unless you want to raise the bar even further and say it hasn't sold 5 mil to be successful or something.
http://www.vgchartz.com/game/70956/bayonetta-2/Global/
Also looking at Capcom's hack n' slash titles has Monster Hunter (A form, read, form of hack n' slash) has the latest one selling at 4.1m.
The genre itself is nowhere near as dormant/dead as say the RTS genre is, but I doubt it needs a Sony exclusive to bring it back from where it is, I don't know why it can only be a Sony ip at that since we will have other hack n' slash games in the future that can also turn out decent.
Also as others have already pointed out, GoW isn't primarily defiend as a hack n' slash game.
Mankind, in its arrogance and self-delusion, must believe they are the mirrors to God in both their image and their power. If something shatters that mirror, then it must be totally destroyed.
